As mentioned above, more dilute developer will exhaust after fewer prints, and even just from standing because more water equals more oxygen. If you were to print a large number of prints you may have problems getting the prints you made later to look like the earlier prints. Some developers produce warmer tones in higher dilution, but it differs from developer to developer. And papers differ. For instance, Ilford RC has developer activators incorporated into the emulsion, (for machine processing) where, as far as my experience suggests, Foma RC does not. Therefore Foma RC may not take to higher dilutions as readily as Ilford. In the end, your eye should tell you.
I suggest an A/B comparison. Next time you print, start with a small amount of 1:9, make a print, and then dilute it to 1:18, and make another print with your adjusted developing time. Compare them when dry.